This application claims the benefit under 35 U.S.C. xc2xa7 365 of International Application PCT/EP99/05450 filed Jul. 30, 1999, which claims the benefit of German Application No. 198 35 803.2, filed Aug. 7, 1998.
The invention relates to a method and an apparatus for convergence correction in a television receiver, in particular in a projection colour television receiver.
Convergence correction devices serve to correct parameters in the raster deflection, for example the correction of north/south and east/west distortions, of pin-cushion distortion, of non-linearities in the deflection and other geometric errors in the horizontal or vertical direction. A particular field of application is convergence correction in the case of a projection television receiver, in which the pictures of three monochromatic picture tubes are projected onto a picture area. Here, television receiver is taken to mean any device with electronic, rasterized picture reproduction. The receiver can be fed by a television broadcast signal or also, as a pure monitor, by a RGB signal, an FBAS signal or, separately, with a luminance signal and a colour subcarrier from an arbitrary video signal source.
The correction of the deflection parameters is undertaken with the aid of digital correction values which are stored in a convergence circuit. The stored correction values are converted in a digital/analogue converter into an analogue control signal and emitted to the first input of an output amplifier whose output is connected to the first terminal of the respectively assigned convergence coil. The second terminal of the convergence coil is connected, on the one hand, to frame potential via a measuring shunt and, on the other hand, to the second input of the output amplifier via a feedback loop. It is ensured in this way that a current corresponding to the analogue control signal is impressed on the convergence coil. Series resistances, for example due to the resistance of conductor tracks, can have the effect that the measuring shunt is no longer at frame potential, but at a potential differing therefrom. The effect of this is that the current in the convergence coil no longer corresponds exactly to the convergence correction value, but deviates from the desired value.
Starting from here, it is the object of the invention to specify a method in which the difficulties described at the beginning and their effects no longer occur in a troublesome fashion.
An advantage of the method according to the invention is that the crosstalk of one convergence channel onto the other convergence channels is compensated by a simple electrical measurement. It is possible in this way for the convergence, for example in a projection television receiver, to be balanced substantially more accurately and quickly, and this leads, on the one hand, to an improved picture quality and, on the other hand, to reduced production costs, because the mutual influencing of the convergence channels is eliminated.
The object is also achieved by a method based on an optical measurement of the crosstalk of one convergence channel into the other convergence channel. The same advantages as with the prior method are achieved with this method. Moreover, it is possible using the optical measuring method also to take account during compensation of such forms of crosstalk which cannot be embodied in the form of a measurable electric voltage. This holds, for example, for stray magnetic fields which are distributed spatially such that although they influence the electron beam of a picture tube they do not cause any measurable induced voltage.
The method according to the invention can be carried out both with optical sensors which are arranged on the picture screen of the television receiver, and by means of an electronic camera.
Furthermore, it is an object of the present invention to provide an apparatus for improved convergence correction.